Although contact lenses are classified as medical devices, there are many kinds, and they are sold in many places, so many people buy them according to the degrees of frame glasses. In fact, this is very irregular. The degree of contact lenses is not exactly the same as the actual degree.
Frame glasses are located in front of the eyes, while contact lenses are directly attached to the cornea surface. The distance between the two lenses and the apex of cornea directly leads to the difference in degrees. According to the principle of optics, the degree of myopia contact lenses is slightly lower than that of frame glasses, while hyperopia is the opposite.
The distance between the frame glasses and the apex of cornea is 12-13MM. For every 1.00d increase, the object image will be enlarged or reduced by 2%. If the degree of myopia is less than 400 degrees, the difference between the two can be ignored, but the higher the degree of myopia is, the more obvious the difference will be. If you wear it in the same degree, it may cause serious fatigue to the eyes, and even increase the degree.
There is a specific conversion formula between contact lenses and frame glasses, that is, contact lens diopter = frame glasses diopter ÷ (1-0.012 × frame glasses diopter). Among them, one diopter is equal to the usual 100 degrees, the diopter of myopia is negative, the diopter of hyperopia is positive, 0.012 is the distance between the frame glasses and the highest point of black eyeball, and the unit is meter. This formula is not easy to remember, so in most cases, we will list tables of all degrees for easy searching.
In addition, in most cases, people's binocular degrees are not the same, so it is necessary to distinguish the left and right eyes when saving the lenses, so as to avoid the adverse effect of cross wearing on the eyes.